For various applications, methods are needed for determining the material constitution of a sample. One of known methods is laser-induced breakdown spectroscopy (LIBS), which involves focusing a laser beam onto a surface of the sample with a high enough power density to transform a small part of the sample material into a state of plasma. Optical emissions from the plasma plume are collected with light collection optics, and the spectral distribution (i.e. intensity as a function of wavelength) of the collected optical emissions is analysed in a spectrometer that produces information in electronic form describing the spectral distribution. Since atomic and molecular constituents of sample materials have characteristic optical emission spectra, the information produced by the spectrometer forms a kind of a fingerprint of the sample material, revealing the constituents of that part of the sample onto which the laser beam was focused.
The sample may in principle be solid, liquid or gaseous. In the case of a gaseous sample the concept of a “surface” of the sample does not exist, but the laser beam is just focused into the gaseous sample.
LIBS is sometimes also referred to as OES (optical emission spectroscopy), although to be quite exact, the latter is a somewhat wider term and may be understood to cover all kinds of optical emission measurements, irrespective of the mechanism that was used to generate the optical emissions.
Prior art publications that describe LIBS measurements are at least U.S. Pat. No. 5,583,634 and U.S. Pat. No. 6,801,595, of which the latter describes the combination of a LIBS measurement with an XRF (X-ray fluorescence) measurement in the same measurement apparatus. A drawback of the known LIBS measurement devices is certain clumsiness and limited applicability to field use. Traditionally LIBS has been considered to be applicable under laboratory conditions only.
An objective of the present invention is to present a LIBS measurement arrangement and devices that are practical to handle and applicable to field use. Another objective of the invention is to enable LIBS measurements of sample forms and locations that would be difficult or impossible to reach with conventional LIBS measurement devices.
The objectives of the invention are achieved by including all essential components of a LIBS measuring arrangement into a single hand-held unit. Certain further objectives of the invention are easiest to reach by using a passive probe that contains the so-called Q-switch, focusing optics and light collection optics.
An apparatus according to the invention for performing laser-induced breakdown spectroscopy is characterised in that it comprises:                a handheld unit,        a pump laser with a controller,        a combination of a solid laser medium and a Q-switch configured to receive a laser beam from said pump laser,        focusing optics configured to focus laser pulses from said combination to a sample,        light collection optics configured to collect light from plasma induced of sample material by focused laser pulses,        a spectrometer configured to receive collected light from said light collection optics and to produce information describing a spectral distribution of such light, and        a power source configured to deliver electric power to other parts of the apparatus;wherein said pump laser, said combination, said focusing optics, said light collection optics, said spectrometer and said power source are parts of said handheld unit.        
The exceptionally high power density that is needed to create plasma is reached by Q-switching, most typically passive Q-switching. It involves using a piece of optical gain medium in connection with a saturable absorber, also known as the passive Q-switch. A saturation effect in the absorber leads to a rapid reduction of resonator loss, so that energy temporarily stored in the gain medium is instantaneously extracted in the form of a laser pulse. The cycle of storing and releasing energy is repeated at a rate determined by the pumping power and the characteristics of the saturable absorber.
Passively Q-switched pulse lasers have been considered to only be suitable to benchtop analysers, because their energy consumption has been relatively high. However, in the course of the development work leading to the present invention it was found that certain means may be applied to significantly reduce the energy consumption. Using Nd:YLF (Neodymium (3+)-doped Yttrium Lithium Fluoride) as the active (gain) medium leads to better efficiency, higher pulse energy and shorter pulse duration, which means that in order to create the same amount of plasma, the electric power needed for the pump laser can be smaller than with e.g. a corresponding Nd:YAG (Neodymium (3+)-doped Yttrium Aluminum Garnet) gain medium. Wavelength locking can be used to stabilize the output wavelength of the laser diode, which substantially eliminates temperature-dependent wavelength drift. This way the power-intensive active temperature control of the pump laser can be completely avoided or at least limited to only compensating for the largest deviations from a nominal operating temperature.
The (passive) Q-switch, focusing optics and light collection optics may be placed in a separate probe part, with an optical fiber cable connecting it to a main part of the measurement device. Such a separate probe part may be very sleek in appearance, so that samples and locations that would be impossible or inconvenient to reach with the whole hand-held measurement apparatus can still be reached with the probe.
The exemplary embodiments of the invention presented in this document are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this document as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated.
The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.